The research proposed here is directed toward developing an understanding of the mechanisms of human visual sensitivity. Specifically, the proposed research will investigate, within both the rod and cone systems, the mechanisms underlying, (1) intensity discrimination in the dark-adapted eye, (2) the effects of background adaptation on intensity discrimination and apparent brightness, and (3) the effects of adaptation on spatial vision. On the basis of our previous work in areas (1) and (2), we have developed a quantitative theory of intensity discrimination and adaptation that is closely aligned with current knowledge of the electrophysiology and retinal neurons. This theory consists of a cascade of linear and static nonlinear stages. A number of psychophysical experiments will be carried out to extend our knowledge about areas (1) - (3) above and to test and extend our current theory. These include measurement of (a) increment- threshold functions at very high flashed-background levels in the dark- adapted rod and cone systems, (b) rod increment-threshold functions under various fixed levels of light adaptation, (c) brightness-matching functions at fixed levels of light adaptation or in the rod and cone systems, (d) increment-threshold functions during very early dark and light adaptation, (e) the sensitization effect during early dark adaptation and (f) contrast sensitivity functions (CSF's) under fixed states of light adaptation.